Method for reproducing in vitro the Proteolytic activity of the NS3 protease of hepatitis C virus (HCV)

ABSTRACT

This is a method for reproducing in vitro the serine protease activity associated with the HCV NS3 protein, that comprises using both of the sequences contained in NS3 and the sequences contained in NS4A. This method takes advantage of the ability of the HCV NS4A protein, or sequences contained therein, to act as a cofactor of the serine protease activity or more generally of the enzymatic activities associated with NS3. Optimal serine protease activity is obtained when NS4A is present in a molar ratio of at least 1:1 with NS3. NS3 and NS4A can also be incorporated in the reaction mixture as NS3-NS4A precursor, as this precursor will generate, by means of an autoproteolytic event, equimolar amounts of NS3 and NS4A. It is also possible to mutate the cleavage site between NS3 and NS4A in a procursor, so that NS4A remains covalently The sequences that do not influence the proteolytic activity of NS3 can subsequently be removed from protease activity.

DESCRIPTION

This present invention has as its subject a method for reconstituting the serine protease activity associated with the HCV NS3 protein, which makes use of the ability of the HCV protein NS4A, or sequences contained therein, to act as a cofactor of the serine protease activity or more generally speaking of enzymatic activities associated with NS3.

As is known, the hepatitis C virus (HCV) is the main etiological agent of non-A, non-B hepatitis (NANB). It is estimated that HCV causes at least 90% of post-transfusional NANB viral hepatitis and 50% of sporadic NANB hepatitis. Although great progress has been made in the selection of blood donors and in the immunological characterization of blood used for transfusions, there is still a high level of acute HCV infection among those receiving blood transfusions (one million or more infections every year throughout the world). Approximately 50% of HCV-infected individuals develop cirrhosis of the liver within a period that can range from 5 to 40 years. Furthermore, recent clinical studies suggest that there is a correlation between chronic HCV infection and the development of hepatocellular carcinoma.

HCV is an enveloped virus containing an RNA positive genome of approximately 9.4 kb. This virus is a member of the Flaviviridae family, the other members of which are the flaviviruses and the pestiviruses. The RNA genome of HCV has recently been mapped. Comparison of sequences from the HCV genomes isolated in various parts of the world has shown that these sequences can be extremely heterogeneous. The majority of the HCV genome is occupied by an open reading frame (ORF) that can vary between 9030 and 9099 nucleotides. This ORF codes for a single viral polyprotein, the length of which can vary from 3010 to 3033 amino acids. During the viral infection cycle, the polyprotein is proteolytically processed into the individual gene products necessary for replication of the virus. The genes coding for HCV structural proteins are located at the 5'-end of the ORF, whereas the region coding for the non-structural proteins occupies the rest of the ORF.

The structural proteins consists of C (core, 21 kDa), E1 (envelope, gp³⁷) and E2 (NS1, gp61). C is a non-glycosylated protein of 21 kDa which probably forms the viral nucleocapsid. The protein E1 is a glycoprotein of approximately 37 kDa and it is believed to be a structural protein for the outer viral envelop. E2, another membrane glycoprotein of 61 kDa, is probably a second structural protein in the outer envelope of the virus.

The non-structural region starts with NS2 (p24), a hydrophobic protein of 24 kDa whose function is unknown. NS2. A protein of 68 kDa which follows NS2 in the polyprotein, is predicted to have two functional domains: a serine protease domain in the first 200 amino-terminal amino acids, and an RNA-dependent ATPase domain at the carboxy terminus. The gene region corresponding to NS4 codes for NS4A (p6) and NS4B (p26), two hydrophobic proteins of 6 and 26 kDa, respectively, whose functions have not yet been clarified. The gene corresponding to NS5 also codes for two proteins, NS5A (p56) and NS5B (p65), of 56 and 65 kDa, respectively. In amino acid sequence present in all the RNA-dependent RNA polymarases can be recognized within the NS5 regions. This suggests that the NS5 region contains parts of the viral replication machinery.

Various molecular biological studies indicate that the signal peptidase, a protease associated with the reticulum of the host cell, is responsible for proteolytic processing in the non-structural region, that is to say at sites C/E1, E1/E2 and E2/NS2. The serine protease contained in NS3 is responsible for cleavage at the junctions between NS3 and NS4A, between NS4A and NS4B, between NS4B and NS5A and between NS5A and NS5B. In particular it has been found that the cleavage performed by this serine protease leaves a residue of cysteine or threonine on the amino-terminal side (position P1) and an alanine or serine residue on the carboxy-terminal side (position P1') of the scissile bond. A second protease activity of HCV appears to be responsible for the cleavage between NS2 and NS3. This protease activity is contained in a region comprising both part of NS2 and the part of NS3 containing the serine protease domain, but does not use the same catalytic mechanism.

In the light of the above description, the NS3 protease is considered a potential target for the development of anti-HCV therapeutic agents. However, the search for such agents has been hampered by the evidence that the serine protease activity displayed by NS3 in vitro is too low to allow screening of inhibitors.

It has now been unexpectedly found that this important limitation can be overcome by adopting the method according to the present invention, which also gives additional advantages that will be evident from the following.

According to the present invention, the method to reproduce in vitro the proteolytic activity of the protease NS3 of HCV is characterized by using in the reaction mixture, both sequences contained in NS3 and sequences contained in NS4A.

Optimal serine protease activity is obtained when NS4A is present in a ration of 1:1 with NS3.

NS3 and NS4A can also be incorporated in the reaction mixture as NS3-NS4A precursor, as this precursor will generate, by means of an autoproteolytic agent, equimolar amounts of NS3 and NS4A.

It is also possible to mutate the site of cleavage between NS3 and NS4A, in a precursor, so that NS4A remains covalently bound to NS3. The sequences that do not influence the proteolytic activity of NS3 can subsequently be removed from this non-proteolyzable precursor.

The invention also extends to a new composition of matter, characterized in that it comprises proteins whose sequences are described in SEQ ID NO:1 and SEQ ID NO:2 or sequences contained therein or derived therefrom. It is understood that these sequences may vary in different HCV isolates, as all the RNA viruses show a high degree of variability. This new composition of matter has the proteolytic activity necessary to obtain the proteolytic maturation of several of the non-structural HCV proteins.

The present invention also has as its subject the use of these compositions of matter in order to prepare an enzymatic assay capable of identifying, for therapeutic purposes, compounds that inhibit the enzymatic activity associated with NS3, including inhibitors of the interaction between NS3 and NS4A.

Up to this point a general description has been given of the present invention. With the aid of the following examples, a more detailed description of specific embodiments thereof will now be given, in order to give a clearer understanding of its objects, characteristics, advantages and method of operation.

The figure illustrates plasmid vectors used in the method to activate the HCV NS3 protease in cultivated cells and in vitro (example 1 and example 2).

EXAMPLE 1 Method of Activation of HCV NS3 Serine Protease in Cultivated Cells

Plasmid vectors were constructed for expression of NS3, NS4A and other non-structural HCV proteins in HeLa-cells. The plasmids constructed are schematically illustrated in FIG. 1. Selected fragments of the cDNA corresponding to the genome of the HCV BK isolate (HCV--BK) were cloned downstream of the promoter of the bacteriophage T7 in the plasmid vector pCite-1^(R) (Novagen). This expression vector contains the internal ribosome entry site of the encephalomyocarditis virus, so as to guarantee an effective translation of the messenger RNA transcribed from promoter T7, even in the absence of a CAP structure.

The various fragments of HCV--BK cDNA were cloned into the plasmid pCite-1^(R) using methods known in molecular biology practice. pCite(NS3) contains the portion of the HCV--BK genome comprised between nucleotides 3351 and 5175 (amino acids 1007-1615 of the polyprotein). pCite(NS4B/5A) contains the portion of the HCV--BK genome comprised between the nucleotides 5652 and 7467 (amino acids 1774-2380). pCite(NS3/4A) contains the portion of the HCV--BK genome comprised between the nucleotides 3711 and 5465 (amino acids 991 and 1711). pCiteNS4A) contains the portion of the HCV--BK genome comprised between the nucleotides 5281 and 5465 (amino acids 1649-1711). pCite(NS5AB) contains the portion of the HCV--BK genome comprised between the nucleotides 6224 and 9400 (amino acids 1965-3010). The numbering given above agrees with the sequences for the genome and the polyprotein given for HCV--BK in Takamizawa et al, Structure and organization of the hepatitis C virus genome isolated from human carriers, (1991), J. Virol. 65, 1105-1113.

In order to obtain efficient expression of the various portions of the HCV polyprotein, the HeLa cells were infected with vTF7-3, a recombinant vaccinia virus which allows synthesis of the RNA polymerase of the bacteriophage T7 in the cytoplasm of infected cells. These cells, after infection, were then transfected with plasmid vectors selected from among those described in figure. The HeLa cells thus infected and transfected were then metabolically labelled with ³⁵ S!methionine and the recombinant proteins encoded by the various plasmids could be identified by immunoprecipitation with polyclonal rabbit antibodies that recognize NS3, NS4 or NS5A. The method described in the present example for analysis of recombinant HCV proteins has already been described in L. Tomei et al, "NS3 is a serine protease required for processing of hepatitis C virus polyprotein", J. Virol. (1993) 67, 1017-1026 and in the bibliography mentioned therein.

By transfecting the plasmid pCite(NS3) into the HeLa cells infected with vTF7-3, it is possible to observe the synthesis of a protein containing the catalytic domain of the HCV NS3 protease. pCite(NS4B5A) coder for a portion of the HCV polyprotein containing a peptide bond, at the junction between NS4B and NS5B, which would be expected to be hydrolyzed by the serine protease activity associated with NS3. However, when pCiteNS3 is cotransfected with pCiteNS4B5A, there is no evidence of proteolytic cleavage. Conversely, when the NS3 serine protease domain is expressed in combination with NS4A the proteolytic cleavage of the precursor encoded by pCite(NS4B5A) can take place normally. Coexpression of the NS3 serine protease domain and 4A can be achieved, for example, by transfection with equimolar amounts of the plasmids pCite(NS3) and pCite(NS4A), by transfection of a plasmid coding for a precursor containing both NS3 and NS4A pCite(NS34A)!, or by transfection of a derivative of the latter plasmid to which all the sequences that are not relevant for proteolysis have been deleted pCite(NS3Δint1237-1633)!. NS4A expressed transiently in HeLa cells can thus activate the proteolytic activity associated with NS3, which otherwise would not be seen.

EXAMPLE 2 Method for Activation of the HCV Serine NS3 Protease in an In Vitro Translation Assay

The plasmids described in FIG. 1 can also be used for in vitro synthesis of mRNA coding for the respective HCV proteins using the purified RNA polymerase enzyme of the phage T7 (Promega).

Generally the plasmids derived from pCite-1^(R) were linearized using suitable restriction enzymes and transcribed using the protocols supplied by the manufacturer (Promega). These synthetic mRNA, could later be used to synthesize the corresponding proteins in extracts of rabbit reticulocytes in the presence of canine pancreas microsomal membranes. The reticulocyte extracts, the canine pancreas microsomal membranes, like all the other material required, were purchased from Promega, which also supplied the instructions for the in vitro protein syntheses process described above.

Programming the in vitro translation mixture with mRNA transcribed from pCite(NS3) it is possible to observe synthesis of a protein with the expected molecular weight (68 kDa) containing the entire NS3 serine protease domain. The mRNA transcribed from pCite(NS5AB) guides the synthesis of a precursor of 115 kDa which contains NS5A and NS5B and is thus a substrate for the proteolytic activity associated with NS3.

However, when the two proteins, containing the NS3 serine protease domain and the substrate with the site corresponding to the junction between NS5A and NS5B, are synthesized in the same reaction mixture, there is no clear evidence of the proteolytic activity of NS3.

On the contrary, the mRNA transcribed from pCite(NS34A) is translated into a precursor protein of approximately 76 kDa which self-processes proteolytically in vitro to give equimolar amounts of two proteins of 70 kDa and 6 kDa, containing NS3 and NS4A, respectively.

If, in addition to the mRNA transcribed from pCite(NS34A), the mRNA transcribed from pCite(NS5AB) is included in the in vitro translation mixture, there can be observed, in addition to the self-proteolysis at the site between NS3 and NS4A, the generation of two new proteins of 56 kDA and 65 kDA which contain NS5A and NS5B, respectively. These proteins represent the product of proteolysis of the precursor containing NS5A and NS5B by NS3. Similarly, the 56 kDa and 65 kDa protein products, generated proteolytically from the NS5AB precursor, are obtained if the mRNA transcribed from pCite(NS3Δint1237-1635) is cotranslated with the mRNA translated from pCite(NS5AB).

This result can be summarized by stating that, in vitro, the protease domain of NS3 alone is not capable of exhibiting protease activity on a substrate containing NS5A and NS5B. However, the serine protease activity of NS3 becomes evident if another protein sequence containing NS4A is present in addition to the NS3 protease domain.

EXAMPLE 3 Method of Activation of the HCV NS3 Protease Using a Synthetic Peptide Containing NS4A Sequences

A synthetic peptide containing the sequence SEQ ID NO:3 was synthesized on solid phase. This sequence is derived from the C-terminal portion of Seq ID NO:2. Synthesis of the peptide took place on solid phase according to processes known to those operating in this field. In this peptide, the carboxy terminal cysteine has been replaced with alpha-aminobutyric acid (Abu).

This peptide was added to an in vitro translation mixture simultaneously programmed with the mRNAs transcribed from the plasmids pCite(NS3) and pCite(NS5AB).

It was thus possible to observe the proteolytic activity associated with the serine protease domain of NS3, which resulted in the proteolytic cleavage of the substrate in the two products containing the proteins NS5A and NS5B. This activity is dependent on the simultaneous presence of the NS3 serine protease domain and the synthetic peptide with the sequence SEQ ID NO:3.

EXAMPLE 4 Method of Assay of a Recombinant HCV NS3 Serine Protease on a Peptide Substrate

The plasmid pT7-7 NS3(1027-1206), described in FIG. 1 and in Example 4, was constructed in order to allow expression in E. coli of the protein fragment comprised between amino acid 1 and amino acid 180 of SEQ. ID No:1. Such fragment contains the serine protease domain of NS3, as determined experimentally. The fragment of HCV cDNA coding for NS3 fragment just described was cloned in the pT7-7 plasmid, an expression vector that contains the T7 RNA polymerase promoter φ 10 and the translation start site for the T7 gene 10 protein (Studier and Moffatt, Use of bacteriophage T7 RNA polymerase to direct selective high-level expression of cloned genes, (1986), J. Mol Biol. 189, p. 113-130). The cDNA fragment coding for the NS3 serine protease domain as defined above was cloned downstream of the bacteriophage T7 promoter and in frame with the first ATG codon of the T7 gene 10 protein, using methods that are known to the molecular biology practice. The pT7-7 plasmid also contains the gene for the β-lactamase enzyme, which can be used as a marker of selection of E. coli cells transformed with plasmids derived with pT7-7.

The plasmid pT7--7 NS3(1027-1206) is then transformed in the E. coli strain BL21(DE53), which is normally employed for high-level expression of genes cloned into expression vectors containing T7 promoter. In this strain of E. coli, the T7 gene polymerase is carried on the bacteriophage λDE53, which is integrated into the chromosome of BL21. Expression from the gene of interest is induced by addition of isopropylthiogalactoside (IPTG) to the growth medium according to a procedure that has been previously described (Studier and Moffatt, Use of bacteriophage T7 RNA polymerase to direct selective high-level expression of cloned genes, (1986), J. Mol Biol- 189, p, 113-130).

The recombinant NS3 fragment containing the serine protease domain could be purified from E. coli BL21(DE53) transformed with the plasmid pT7-7 NS3(1027-1206) by the procedure summarized below.

In brief, E. coli BL21(DES3) cells harboring the pT7-7 NS3(1027-1206) plasmid were grown at 37° C. to an optical density at 600 nm of around 0.8 absorbance units. Thereafter, the medium was cooled down to 22° C. and production of the desired protein induced by addition of IPTG to a final concentration of 0.4 mM. After 4-6 hours at 22° C. in the presence of IPTG, cells were harvested and lysed by means of a French-pressure cell in a buffer containing 20 mM sodium phosphate pH 6.5, 0.5% (w/v) (3- (3-cholamidopropyl)-dimethylammonio! 1-propanesulfonate (CHAPS), 50% (v/v) glycerol, 10 mM dithiothreitol and 1 mM EDTA (lysis buffer). The cell debris was removed by centrifugation (1 hour at 12000×g) and the resulting pellet resuspended in lysis buffer, digested with DNAse I, re-homogenized and re-centrofuged as described above. S-Sepharose Fast Flow ion exchange resin (Pharmacia) pre-equilibrated in lysis buffer was added to the pooled supernatants (30% v/v) and the slurry was stirred for 1 hour at 4° C. The resin was sedimented and washed extensively with lysis buffer and poured into a chromatography column. The NS3 protease was deluted from the resin by applying a 0-1M NaCl gradient. The protease-containing fractions equilibrated with 50 mM sodium phosphate buffer pH 7.5, 10% (v/v) glycerol, 0.5% (w/v) CHAPS and 2 mM dithiothreitol. The protein was 90-95% pure after this step. Purification to >98% was achieved by subsequent chromatography on Heparin Sepharose equilibrated with 50 mM Tris pH 7.5, 10% (v/v) glycerol, 0.5% (w/v) CHAPS and 2 mM dithiothreitol. Elution of the NS3 protease from this column was achieved by applying a linear 0-1M NaCl gradient.

The concentration of the purified protein was determined by the Bio-Rad protein assay (Bio-Rad cat. 500-0006).

The recombinant NS3 serine protease produced according to the above procedure in E. coli could be assayed for activity by cleaving a substrate that provides detectable cleavage products. The signal is preferably detectable by colorimetric or fluorometric means. Methods such as HPLC and the like are also suitable.

For example, we used, as a substrate, synthetic peptides corresponding to the NS4A/4B junction of the HCV polyprotein.

The activity assay is performed by incubating 5-1000 μM substrate and 0.05-1 μM protease in buffer containing 25 mM Tris/HCl pH 7.5, 3 mM dithiothreitol, 0.5% (w/v) CHAPS and 10% (v/v) glycerol for 1-3 hours at 22° C. The reaction is stopped by addition of trifluoracetic acid to yield a final concentration of 0.1% (w/v).

The reaction products are then separated by HPLC on a C18 reverse phase column and quantitated according to their absorbance of the far UV light.

The proteolytic activity displayed by recombinant NS3 serine protease purified from E. coli is very low when the activity assay is performed as described above. However, we found that increasing amounts of the synthetic peptide described in SEQ ID NO:3 stimulate the proleolytic activity of the recombinant NS3 serine protease up to 20-fold. Maximal activity is reached when the recombinant NS3 serine protease and the synthetic peptide are present in equimolar amounts.

The assay described above can be used for the search of protease inhibitors. Because the activity of NS3 protease in such assay depends on the interaction of the NS3 serine protease domain with amino acid sequences derived from NS4A, it is also possible, by using the assay described above, to search for antagonists of the interaction between NS3 and NS4A that will ultimately inhibit the proteolytic activity associated with NS3.

EXAMPLE 5 Detailed Construction of the Plasmids in the Sole Figure

pCite(NS3) contains the portion of the HCV--BK genome comprised between nucleotides 3351 and 5175 (amino acids 1007-1615 of the polyprotein). Construction of this plasmid has been described in L. Tomei et al, "NS3 is a serine protease required for processing of hepatitis C virus polyprotein", J. Virol. (1993) 67, 1017-1026.

pCite(NS4B/5A) was obtained by cloning a ScaI-BamHI fragment derived from the plasmid pCite(NS4-5), described in Tomei et al, into pCite(NS3) that was previously digested with MscI and BamHI. pCite(NS4B/5A) contains the portion of the HCV genome comprised between nucleotides 5652 and 7467 (amino acids 1774-2380 of the polyprotein).

pCite(NS5AB) codes for a protein that comprises the sequence from amino acid 1965 to amino acid 3010 of the HCV--BK polyprotein. To construct this plasmid, the plasmid pCite(SX) described in Tomei et al (1993), supra, was first digested with AseI and treated with the Klenow fragment of the DNA polymerase. After inactivation of the Klenow enzyme, the plasmid was digested with XbaI. The resulting cDNA fragment, containing the region between nucleotides 6224 and 9400, was purified and inserted into the BstXI and XbaI sites of the vector pCite-1^(R), after blunting the end generated by BstXI with T4 DA polymerase.

pCite(NS3/4A) was obtained as follows. A cDNA fragment, corresponding to the region between nucleotides 3711 and 5465 of the HCV--BK genome, was synthesized by means of polymerase chain reaction (PCR) using sequence-specific oligonucleotides as primers. A UAG stop codon was suitably included in the antisense oligonucleotide. After PCR amplification, the resulting cDNA was cleaved at the 5' end with SAlI and the product of 750 pairs of bases cloned directionally into the SalI and NheI sites of the plasmid pCite(SX), after blunt-ending the NheI end with the Klenow fragment of the DNA polymerase. The resulting plasmid codes for the portion of HCV--BK polyprotein comprised between amino acids 991 and 1711.

For the construction of pCite(NS4A), a cDNA fragment, corresponding to the region between the nucleotides 5281 and 5465 of the HCV-BK genome (amino acids 1649-1711), was obtained by polymerase chain reaction (PCR) amplification with sequence-specific oligonucleotides as primers. The cDNA resulting from the PCR amplification was subsequently cloned into the BstxI and StuI sites of the plasmid pCite-1^(R), after blunt-ending the BstXI digested end with the DNA polymerase of the bacteriophage T4.

pCite(NS3Δint1237-1635) is a derivative of pCite(NS3/4A) from which all the sequences comprised between nucleotide 4043 and nucleotide 5235 have been deleted. It was obtained by digesting pCite(NS3/4A) with BsteII and partially with ScaI. The fragment containing the deletion of interest was then circularised by use of T4 DNA ligase. This plasmid codes for a protein that has the same amino- and carboxy-terminal ends as that encoded by pCite(NS3/4A), but all the amino acid residues comprised between amino acid 1237 and amino acid 1635, experimentally found to be dispensible for the serine protease NS3 activity, have been deleted.

pT7-7 NS3(1027-1206)! contains the HCV sequence from nucleotide 3411 to nucleotide 3951, encoding the HCV NS3 fragment comprised between amino acid 1027 and amino acid 1206. In order to obtain this plasmid, a DNA fragment was generated by amplification of HCV cDNA by the polymerase chain reaction (PCR). The cDNA fragment obtained by PCR was phosphorylated, digested with Nde I and subsequently cloned downstream of the bacteriophage T7 promoter, following immediately the first ATG codon of the T7 gene 10 protein in the vector pT7-7 previously digested with Nde I and Sma I (Studier and Moffatt, Use of bacteriophage T7 RNA polymerase to direct selective high-level expression of cloned genes, (1986), J. Mol. Biol. 189, p. 113-130). It is to note that an amber codon was inserted immediately following the HCV-derived sequence.

    __________________________________________________________________________     SEQUENCE LISTING                                                               (1) GENERAL INFORMATION:                                                       (iii) NUMBER OF SEQUENCES: 3                                                   (2) INFORMATION FOR SEQ ID NO:1:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 631 amino acids                                                    (B) TYPE: amino acid                                                           (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: protein                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                        AlaProIleThrAlaTyrSerGlnGlnThrArgGlyLeuLeuGlyCys                               151015                                                                         IleIleThrSerLeuThrGlyArgAspLysAsnGlnValGluGlyGlu                               202530                                                                         ValGlnValValSerThrAlaThrGlnSerPheLeuAlaThrCysVal                               354045                                                                         AsnGlyValCysTrpThrValTyrHisGlyAlaGlySerLysThrLeu                               505560                                                                         AlaAlaProLysGlyProIleThrGlnMetTyrThrAsnValAspGln                               65707580                                                                       AspLeuValGlyTrpProLysProProGlyAlaArgSerLeuThrPro                               859095                                                                         CysThrCysGlySerSerAspLeuTyrLeuValThrArgHisAlaAsp                               100105110                                                                      ValIleProValArgArgArgGlyAspSerArgGlySerLeuLeuSer                               115120125                                                                      ProArgProValSerTyrLeuLysGlySerSerGlyGlyProLeuLeu                               130135140                                                                      CysProPheGlyHisAlaValGlyIlePheArgAlaAlaValCysThr                               145150155160                                                                   ArgGlyValAlaLysAlaValAspPheValProValGluSerMetGlu                               165170175                                                                      ThrThrMetArgSerProValPheThrAspAsnSerSerProProAla                               180185190                                                                      ValProGlnSerPheGlnValAlaHisLeuHisAlaProThrGlySer                               195200205                                                                      GlyLysSerThrLysValProAlaAlaTyrAlaAlaGlnGlyTyrLys                               210215220                                                                      ValLeuValLeuAsnProSerValAlaAlaThrLeuGlyPheGlyAla                               225230235240                                                                   TyrMetSerLysAlaHisGlyIleAspProAsnIleArgThrGlyVal                               245250255                                                                      ArgThrIleThrThrGlyAlaProValThrTyrSerThrTyrGlyLys                               260265270                                                                      PheLeuAlaAspGlyGlyCysSerGlyGlyAlaTyrAspIleIleIle                               275280285                                                                      CysAspGluCysHisSerThrAspSerThrThrIleLeuGlyIleGly                               290295300                                                                      ThrValLeuAspGlnAlaGluThrAlaGlyAlaArgLeuValValLeu                               305310315320                                                                   AlaThrAlaThrProProGlySerValThrValProHisProAsnIle                               325330335                                                                      GluGluValAlaLeuSerAsnThrGlyGluIleProPheTyrGlyLys                               340345350                                                                      AlaIleProIleGluAlaIleArgGlyGlyArgHisLeuIlePheCys                               355360365                                                                      HisSerLysLysLysCysAspGluLeuAlaAlaLysLeuSerGlyLeu                               370375380                                                                      GlyIleAsnAlaValAlaTyrTyrArgGlyLeuAspValSerValIle                               385390395400                                                                   ProThrIleGlyAspValValValValAlaThrAspAlaLeuMetThr                               405410415                                                                      GlyTyrThrGlyAspPheAspSerValIleAspCysAsnThrCysVal                               420425430                                                                      ThrGlnThrValAspPheSerLeuAspProThrPheThrIleGluThr                               435440445                                                                      ThrThrValProGlnAspAlaValSerArgSerGlnArgArgGlyArg                               450455460                                                                      ThrGlyArgGlyArgArgGlyIleTyrArgPheValThrProGlyGlu                               465470475480                                                                   ArgProSerGlyMetPheAspSerSerValLeuCysGluCysTyrAsp                               485490495                                                                      AlaGlyCysAlaTrpTyrGluLeuThrProAlaGluThrSerValArg                               500505510                                                                      LeuArgAlaTyrLeuAsnThrProGlyLeuProValCysGlnAspHis                               515520525                                                                      LeuGluPheTrpGluSerValPheThrGlyLeuThrHisIleAspAla                               530535540                                                                      HisPheLeuSerGlnThrLysGlnAlaGlyAspAsnPheProTyrLeu                               545550555560                                                                   ValAlaTyrGlnAlaThrValCysAlaArgAlaGlnAlaProProPro                               565570575                                                                      SerTrpAspGlnMetTrpLysCysLeuIleArgLeuLysProThrLeu                               580585590                                                                      HisGlyProThrProLeuLeuTyrArgLeuGlyAlaValGlnAsnGlu                               595600605                                                                      ValThrLeuThrHisProIleThrLysTyrIleMetAlaCysMetSer                               610615620                                                                      AlaAspLeuGluValValThr                                                          625630                                                                         (2) INFORMATION FOR SEQ ID NO:2:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 54 amino acids                                                     (B) TYPE: amino acid                                                           (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                        SerThrTrpValLeuValGlyGlyValLeuAlaAlaLeuAlaAlaTyr                               151015                                                                         CysLeuThrThrGlySerValValIleValGlyArgIleIleLeuSer                               202530                                                                         GlyArgProAlaIleValProAspArgGluLeuLeuTyrGlnGluPhe                               354045                                                                         AspGluMetGluGluCys                                                             50                                                                             (2) INFORMATION FOR SEQ ID NO:3:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 34 amino acids                                                     (B) TYPE: amino acid                                                           (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (D) OTHER INFORMATION: /note= "Xaa at position 34 means                        Abu (2- Aminobutyric acid)"                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:                                        GlySerValValIleValGlyArgIleIleLeuSerGlyArgProAla                               151015                                                                         IleValProAspArgGluValLeuTyrGlnGluPheAspGluMetGlu                               202530                                                                         GluXaa                                                                         __________________________________________________________________________ 

We claim:
 1. A method for determining cofactor activity of a candidate compound comprising a fragment of hepatitis C virus (HCV) NS4A protein on HCV NS3 protease, comprising the steps of:(a) providing a reaction mixture comprising NS3 protease and a NS3 protease substrate; (b) introducing the candidate compound to the reaction mixture; and (c) measuring NS3 protease activity to determine whether an increase in protease activity has occurred.
 2. The method according to claim 1, wherein the NS3 protease is a recombinant protein.
 3. A method for determining HCV NS3 protease modulating activity of a candidate compound, comprising the steps of:(a) providing a reaction mixture comprising HCV NS3 protease, HCV NS4A cofactor, and a NS3 protease substrate; (b) introducing a candidate compound to the reaction mixture; and (c) measuring NS3 protease activity to determine whether modulation of protease activity has occurred.
 4. The method according to claim 3, wherein the NS3 protease and the NS4A cofactor are recombinant proteins.
 5. The method according to claim 3, wherein the NS3 protease and the NS4A cofactor are fused together as a single recombinant protein.
 6. The method according to claim 5, wherein a proteolytic site of cleavage between the NS3 protease and the NS4A cofactor is mutated so as to be resistant to proteolytic cleavage between NS3 and NS4A.
 7. The method according to claim 3, wherein the NS4A cofactor is a synthetic peptide.
 8. The method according to claim 3, wherein the NS4A cofactor and NS3 protease are present in a ratio of approximately 1:1.
 9. The method according to claim 3, where the modulation of protease activity is inhibition.
 10. The method according to claim 9, wherein the NS3 protease has a sequence of SEQ ID NO:1 and the NS4A cofactor has a sequence of SEQ ID NO:2.
 11. The method according to claim 3, wherein the NS3 protease has a sequence of SEQ ID NO:1 and the NS4A cofactor has a sequence of SEQ ID NO:2.
 12. A method for determining modulating activity of a candidate compound on HCV NS3 protease/NS4A cofactor interaction, comprising the steps of:(a) providing a reaction mixture comprising HCV NS3 protease, HCV NS4A cofactor, and a NS3 protease substrate. (b) introducing a candidate compound to the reaction mixture; and (c) measuring NS3 protease activity to determine whether modulation of NS3 protease/NS4A cofactor interaction has occurred.
 13. The method according to claim 12, wherein the NS3 protease and the NS4A cofactor are recombinant proteins.
 14. The method according to claim 12, wherein the NS3 protease and the NS4A cofactor are fused together as a single recombinant protein.
 15. The method according to claim 14, wherein a proteolytic site of cleavage between the NS3 protease and the NS4A cofactor is mutated so as to be resistant to proteolytic cleavage.
 16. The method according to claim 12, wherein the NS4A cofactor is a synthetic peptide.
 17. The method according to claim 12, wherein the modulation of NS3 protease/NS4A cofactor interaction is inhibition.
 18. The method according to claim 17, further comprising a step of determining cofactor concentration dependence of the inhibition of NS3 protease/NS4A cofactor interaction.
 19. The method according to claim 17, wherein the NS3 protease has a sequence of SEQ ID NO:1 and the NS4A cofactor has a sequence of SEQ ID NO:2.
 20. The method according to claim 12, wherein the NS3 protease has a sequence of SEQ ID NO:1 and the NS4A cofactor has a sequence of SEQ ID NO:2. 